An IT (French: Isolé Terre)-Grid does not comprise a zero or neutral conductor. Thus, an isolation failure occurring between one of the conductors and ground initially only results in grounding of this conductor. In an IT-Grid, which normally ends on the alternating current side of the inverter at a transformer for feeding the electric energy in, for example, a public or local alternating current grid having a reference to ground, high currents, however, result in case of a connection to ground of a conductor on the alternating current side. This particularly applies to inverters operated at a high frequency. These high currents result in danger to persons and equipment so that, in case of their occurrence, shutting down is due immediately.
A measurement arrangement for determining the isolation resistance of an arrangement comprising a photovoltaic device that is connected via an inverter to a single phase grid having a grounded neutral conductor is known from DE 10 2006 022 686 A1. The known measurement arrangement for determining the isolation resistance with respect to ground may thus only operate with the inverter being deactivated. The isolation resistance is measured by means, of reference resistors that are grounded at one of their ends and that are alternately connected to the two conductors coming from the photovoltaic device on the direct current side of the inverter, whereupon voltages dropping between the two conductors and ground are measured. Alternatively, the currents that flow upon alternately connecting the two conductors to ground may also be measured. In the known measurement arrangement, each individual measurement of a current or a voltage takes several seconds up to minutes, since at first a stable distribution of the electric charges over all capacitances active with respect to ground has to be reached.
A safety device for monitoring an isolation of a DC-voltage bus is known from EP 1 265 076 B1, which, besides other documents, is referenced as prior art in the introductional part of the description of DE 10 2006 022 686 A1. A photovoltaic device that is connected via an inverter and an isolating transformer to an alternating current grid is indicated as one possible application. Due to the isolating transformer the alternating current side of the transformer is also isolated with respect to ground so that monitoring the DC-voltage bus isolation may also happen while the inverter is running, since it does not connect to ground. Monitoring the DC-voltage bus isolation is based here also on the measurement of voltages that drop from the conductors on the direct current side of the inverter over reference resistors that are grounded at one of their ends.
A further monitoring of the isolation of an IT-Grid on the basis of voltage drops over reference resistors is known from EP 0 833 423 A2, in which no application in connection with a photovoltaic device connected to an inverter is described.
A product of the company Dipl.-Ing. W. Bender GmbH & Co. KG having the product designation IRDH 275 is known as a further apparatus. This known apparatus applies a defined voltage with respect to ground to one of the conductors on the direct current side of the inverter at a time and measures the resulting current, which still flows even when a stable distribution of the charges over all capacitances that are active with respect to ground has been reached. This current is a direct measure of the isolation resistance of the respective conductor with respect to ground. This measurement is done at each of the two conductors on the direct current side of the inverter. Additionally, the polarity with respect to ground is inverted between two consecutive measurements at the same conductor to assure the measurement result. Due to the capacitances with respect to ground, which have to be reloaded in case of each new measurement until a stable measurement value is reached, each measurement takes several seconds to minutes. Correspondingly an isolation fault may only be recognized very slowly with this known apparatus and with this known method. However, even small isolation faults at a high isolation resistance are recognized at high accuracy. In the known application of the apparatus of the company Bender, these isolation faults of the IT-grid are also determined on the alternating current side of the inverter as they have an effect via the inverter up to the direct current side of the inverter.
So called GFCIs or ground fault circuit interrupters, which are, according to their English designation “residual current protective device”, also nominated as RCD, separate a monitored alternating current circuit from ground, when a certain differential current strength between the currents flowing off and flowing back is exceeded, which indicates a leakage current towards ground. For determining the differential current strength, the conductors of the monitored AC current circuit are collectively guided through a ring core. The resulting magnetization of the ring core is a result of the sum of the currents through the conductors added up with their correct signs. In case of a short circuit of a conductor of the alternating current circuit with respect to ground, a transient current difference results, which may be easily determined by an induction coil around the ring core and be transformed in a signal for breaking the monitored electrical circuit. The use of ground fold circuit interruptors for ground connection monitoring is generally not possible in an IT-grid (see, for example, http://de.wikipedia.org/wiki/Niederspannungsnetz, and there the text referring to IT-systems in the sub-section “Arten”).
A method and an apparatus for detecting isolation faults of a photovoltaic device with respect to ground are known from US 2002/0105765 A1. For this purpose, a differential current between the input lines of an inverter, which feeds the electric energy from the photovoltaic device into an alternating current grid, is determined. Besides the fault current of interest in case of a connection to ground in the area of the photovoltaic device, the differential current comprises a ground current that occurs, due to the finite capacitances of the photovoltaic device with respect to ground, as a result of potential changes of the input lines with respect to the ground reference of the output lines of the inverter at the switching frequency of the inverter. These ground current components, that flow towards ground on the direct current side of the converter via the capacitances of the photovoltaic device even without occurrence of a fault, are removed from the differential current to singly obtain the fault current signal of actual interest. US 2002/0105765 A1 does not relate to any isolation faults of the output lines of the inverter on its alternating current side.